Security systems are presently limited in their ability to detect contraband, weapons, explosives, and other dangerous objects concealed under clothing. Metal detectors and chemical sniffers are commonly used for the detection of large metal objects and some varieties of explosives, however, a wide range of dangerous objects exist that cannot be detected with these devices. Plastic and ceramic weapons developed by modern technology increase the types of non-metallic objects that security personnel are required to detect; the alternative of manual searching of subjects is slow, inconvenient, and is not well-tolerated by the general public, especially as a standard procedure in, for example, airports.
Further, radiation exposure is an important consideration in X-ray concealed object detection systems. This issue is addressed in the American National Standard “Radiation Safety for Personnel Security Screening Systems Using X-rays” (ANSI/HPS N43.17-2002). This standard permits a radiation exposure of 0.1 microsievert (10 microrem) per scan for security inspection of the general public. It is based on the recommendations of the United States National Council on Radiation Protection (NCRP) in NCRP Report No. 91, “Recommendations on Limits for Exposure to Ionizing Radiation”, 1987. In this report, the NCRP states that the health risk of a radiation exposure of less than 10 microsieverts (1000 microrem) per year is negligible, and efforts are not warranted at reducing the level further. Persons employed in high security or secured facilities, or those who frequently travel by airlines, may be subjected to many security examinations per year. The standard criterion thus assures that an individual inspected less than about 100 times per year will not receive a non-negligible radiation dose.
Conventional systems and methods for detecting objects concealed on persons have limitations in their design and method which prohibit them from achieving both low dose and high image quality which are prerequisites of commercial acceptance. Specifically, conventional prior art systems for people screening are designed such that they detect radiant energy that has been transmitted through the body, scattered from the body, and/or emitted from the body. In addition, in conventional people screening systems, images are produced by body characteristics and any object concealed under the subject's clothing. The system operator then inspects each image for evidence of concealed objects.
An example of such a system is described in U.S. Pat. No. RE 28544, assigned to American Science and Engineering, describes a “radiant energy imaging apparatus comprising: a source of a pencil beam of X-ray radiant energy; radiant energy detecting means defining a curve in fixed relationship to said source; means for scanning with said pencil beam said radiant energy detecting means along said curve to provide an image signal representative of the radiant energy response of the medium in a region traversed by said pencil beam along a path to said detecting means; means for relatively displacing said region and an assembly comprising said source and said detecting means to establish relative translating motion in a direction transverse to a line joining said source and said detecting means to produce a sequence of image signals representative of the radiant energy response of said region in two dimensions; and means responsive to said image signals for producing an image representative of said response.”
U.S. Pat. No. 5,181,234, assigned to the assignee of the present invention, and herein incorporated by reference, discloses “X-ray imaging apparatus for detecting a low atomic number object carried by or on a human body positioned at a distance from said apparatus comprising: x-ray source for producing a pencil beam of X-rays directed toward said human body; scanning means for moving the region of intersection of said pencil beam and said human body over the surface of said human body in a scanning cycle, said scanning cycle being sufficiently short to expose said human body to a low radiation dose; a detector assembly providing a signal representative of the intensity of the X-rays scattered from said human body as a result of being scanned by said scanning means, said detector assembly being disposed on a same side of said human body as said X-ray source and having an active area with dimensions sufficient to receive a substantial portion of said scattered X-rays to provide a coefficient of variation of less than 10 percent in said signal; and display means to presenting characteristics of the detector signal to an operator; wherein said scattered X-rays are distributed across said detector to create an edge effect which enhances edges of said low atomic number object to enable detection.”
In addition, prior art baggage inspection systems include detection means for both transmitted and backscattered X-rays to independently produce signals from the same incident beam. The separate signals may then be used to enhance each other to increase the system's accuracy in recognizing low Z materials. Clearly, with the incident beam being of sufficient energy to provide both transmitted and backscattered signals, the X-ray energy must be relatively high, making such systems undesirable for personnel inspection. An example of such a system is U.S. Pat. No. 4,799,247, assigned to Annis et al., which discloses “a projection imaging system for inspecting objects for highlighting low Z materials comprising: a source of penetrating radiation, means for forming radiation emitted by said source into a beam of predetermined cross-section and for repeatedly sweeping said beam across a line in space, means for moving said object to be imaged relative to said source in a direction perpendicular to said line in space, first radiant energy detector means located to be responsive to radiant energy penetrating said object and emerging from said object, substantially unchanged in direction, for producing first electrical signals, second radiant energy detector means located further from said source than said object and responsive to radiant energy scattered by said object for producing second electrical signals, third radiant energy detector means located closer to said source than said object and responsive to radiant energy scattered by said object for producing third electrical signals, display means responsive to at least a pair of said electrical signals for separately, independently and simultaneously displaying said pair of electrical signals as a function of time”.
As mentioned above, conventional systems and methods have limitations that prohibit them from achieving both low dose and high image quality which are prerequisites of commercial acceptance. In addition, in conventional people screening systems, images are produced by body characteristics and any object concealed under the subject's clothing.
The prior art systems are disadvantageous, however, because they do not adequately detect plastics, ceramics, explosives, illicit drugs, and other non-metallic objects. One reason in particular is that these materials share the property of a relatively low atomic number (low Z). Low Z materials present a special problem in personnel inspection because of the difficulty in distinguishing the low Z object from the background of the subject's body which also has low Z. An inspection system which operates at a low level of radiation exposure is limited in its precision by the small number of X-rays that can be directed against a person being searched. X-ray absorption and scattering further reduces the number of X-rays available to form an image of the person and any concealed objects. In prior art systems, this low number of detected X-rays has resulted in unacceptably poor image quality.
Therefore, what is needed is a method and apparatus that increases the efficiency of a detector to detect electromagnetic radiation and improve the quality of the resultant image generated, thus reducing the overall amount of radiation required.
What is also needed is a method for using an improved radiant energy imaging system with enhanced detection capabilities.